ATP roundup: Murray loses thriller, De Minaur suffers shock defeat
Khachanov, of Russia, fired six aces and benefited from three double faults by his opponent to dispatch the Italian in 88 minutes.
Top-seeded Andrey Rublev of Russia cruised past France's Richard Gasquet 6-3, 6-4. The third seed, Ugo Humbert of France, also sailed through to the quarterfinal round with a 6-4, 6-1 win over Pavel Kotov of Russia.
Fourth-seeded Alexander Bublik had a tougher time in his match against Marton Fucsovics of Hungary. The Kazakhstan native overcame six double faults and rebounded from a first-set misstep in his 2-6, 6-3, 6-4 win.
Also Wednesday, Alexei Popyrin of Australia notched a 7-6, 7-6, 6-1 victory over Frenchman Hugo Grenier, and Finland's Emil Ruusuvuori posted a 6-3, 6-3 win over Christopher O'Connell of Australia.
Former world number one Andy Murray fell in a tough three-set match to Czech 18-year-old Jakub Mensik, who ousted fifth seed Alejandro Davidovich Fokina of Spain in the first round. Mensik needed three hours and 26 minutes to beat Murray 7-6, 6-7, 7-6.
France's Gael Monfils eked out a 4-6, 7-5, 7-6 victory over China's Zhang Zhizhen.
Rio Open
17-year-old Brazilian Joao Fonseca posted the first ATP Tour victory of his career, upsetting seventh-seeded Frenchman Arthur Fils 6-0, 6-4 in the first round in Rio de Janeiro.
Fonseca, the reigning US Open boys champion, is ranked 655 in the world, while Fils is ranked 36th. Fonseca never faced a break point and won 68 per cent of the points on Fils' second serve.
Only one other match was completed on Wednesday in a rain-interrupted slate. Fourth-seeded Francisco Cerundolo of Argentina routed Spain's Albert Ramos-Vinolas 6-2, 6-1.
Los Cabos Open
The United States' Alex Michelsen scored a second-round upset in Los Cabos, Mexico, eliminating third-seeded Alex de Minaur of Australia 6-4, 6-1.
The players' serves dictated the outcome, as Michelsen had a 62 per cent first-serve rate while de Minaur put just 38 per cent of his first serves in play.
Second-seeded Stefanos Tsitsipas of Greece, fourth-seeded Casper Ruud of Norway and eighth-seeded Jordan Thompson of Australia all cruised to straight-set wins, while Portugal's Nuno Borges got past seventh-seeded Max Purcell of Australia 6-3, 6-3. Aleksandar Kovacevic of the United States also advanced.
In night action, top-seeded Alexander Zverev of Germany trounced Japan's Yoshihito Nishioka 6-3, 6-0, and Australia's Thanasi Kokkinakis downed Great Britain's Daniel Evans 6-4, 6-2.